Method for producing crystal plate of gadolinium molybdate

ABSTRACT

A method for producing single crystal plate of gadolinium molybdate which comprises forming a crystal plate of a specified orientation of gadolinium molybdate at the surface of a melt of gadolinium molybdate while giving to the melt a temperature gradient of inner portion of the melt being at a high temperature and of the melt surface being at a lower temperature than that of the inner portion and thereafter cooling the whole melt from 1200*C to 1100*C at a cooling rate of 30*-170*C/min to obtain single crystal of a specified orientation of gadolinium molybdate.

United States Patent [111 3,775,066

Yumoto 1 Nov. 27, 1973 METHOD FOR PRODUCING CRYSTAL 3,562,175 2/1971 Hickok 252/30l.4 R

PLATE 0F GADOLINIUM MOLYBDATE OTHER PUBLICATIONS Hiroshi Yumoto, Kodaira, Japan Assignee: Hitachi, Ltd., Tokyo, Japan Filed: Sept. 27, 1971 Appl. No.: 183,864

Inventor:

Foreign Application Priority Data Sept. 30, 1970 Japan 45/85068 US. Cl. 23/301 SP, 23/305, 423/606 Int. Cl B01] 17/18, C0lg 39/00 Field of Search 23/305; 423/593,

References Cited UNITED STATES PATENTS 11/1924 Kissock 423/593 12/1966 Vogel et a1. 423/263 6/1967 Borchardtl... 423/606 4/1969 Borchardtf. 423/263 POLYCRYSTALLINE MOLYBDATE romeo AT SLOWER COOLING RATE Kestigian, J. Amer. Ceram. Soc., 48, pp. 544-545,

Primary Examiner-Norman Yudkoff Assistant Examiner-R. T. Foster Attorney-Craig, Antonelli & Hill [5 7] ABSTRACT A method for producing single crystal plate of gadolinium molybdate which comprises forming a crystal plate of a specified orientation of gadolinium molybdate at the surface of a melt of gadolinium molybdate while giving to the melt a temperature gradient of inner portion of the melt being at a high temperature and of the melt surface being at a lower temperature than that of the inner portion and thereafter cooling the whole melt from [200C to 1100C at a cooling rate of 30-l70C/min to obtain single crystal of a specified orientation of gadolinium molybdate.

l2 Claims,'7 Drawing Figures SINGLE CRYSTAL DISC MOLYBDATE FORMED AT PREDETERMINED COOLING RATE PATENTED NOV 2 7 i975 SHEET 10F 2 FIG.

OLING SD IIEC M LR AO TFN S I Y M RHR CA n EB LY GLE NOR QMP RATE POLYCRYSTALLINE MOLYBDATE FORMED AT SLOWER COOLING RATE FIG. 2

WAVE LENGTH x I0 3) INVENTOR HIRO HI YUNDTO BY Mtowzfli; b w

ATTORNEY PAIENIEDIIIJV27 ms SHEET 2 F 2 FIG. 3b

FIG. 30

FIG. 5

FIG. 4

0 IO 3b 10v mbqm mmmuoDm 0 IO 20 3O 4O 50 g NEE mwwuuzm THICKNESS OF MOLTEN DIAMETER OF LIQUID mrn VESSEL mm FIG. 6

TIME I OPTIONAL IN VENTOR HIR HI Y 'IOTO w MtOWQQQA, (r HULQ ATTORNEYS METHOD FOR PRODUCING CRYSTAL PLATE OF GADOLINIUM MOLYBDATE BACKGROUND OF THE INVENTION The present invention relates to a method for growing single crystal plate .of gadolinium molybdate.

As a method for producing single crystal of gadolinium molybdate (which is referred to as MOG hereinafter), U.S. Pat. No. 3,437,432 proposed to obtain single crystal from a melt of MOG by Czochralskie method. However, in order to obtain single crystal plate of MOG having a specified orientation, it was necessary to cut out single crystal plate having the desired orientation from single crystal produced by said conventional method and then to subject the crystal plate to surface polishing. Itrequires very accurate and skilled work to cut out single crystal plate of a specified orientation. Furthermore, there is a defect that mechanical strain is caused in crystal in cutting out of the crystal.

Said MOGsingle crystal to which electric field (coercive electric field) or stress(coercive stress) at least equal to the threshold value specific to said crystal is applied exhibits physical properties which are the same as thoseof the crystal whose a.axis and b axis are replaced. Said properties keep a kind of memory state which remains as it is, after the cause (i.e. external electric field or external stress) has been removed. Therefore, many practical-uses of said MOG single crystal are expected with reference to optical characteristic of the crystal and mechanical properties. Therefore, it is necessary to utilize'the crystal plate having a specified orientation in order to offer the MOG for practical use.

The growth rate of MOG crystal is highest at direction l next at l and extremely low at 001 as compared with the first two cases.

Considering that when crystal is allowedto semicircularly grow in free state in melt of MOG, (001 crystal plate, namely, C-plate which is in perpendicular direction to c axis can be allowed to grow, the inventors have made various experiments to fing the following facts.

The melt of MOG can well wet the surface of platinum vessel and has a great sticking force to the platinum surface when it is solidified. Therefore, anattempt to homogeneously single-crystallize the melt of MOG in a platinum vessel has not turned out asuccess'The following is one example thereof.

a. A melt of 25 g of MOG was poured into a platinum boat of 10 mm in width, 80 mm in length and 8 mm in depth to form ahomogeneous polycrystal. Then, when a melting zone of about 10mm in width was transferred from one end to another end of the melt at a velocity of l0-l00'mm/hr, the whole of the melt was not converted into single crystal and only polycrystal containing several crystal grains which stick to platinum wall could be obtained.

b. 14 Grams of MOG in polycrystal form was placed in a Bridgman type crucible having a diameter of 10 mm, a depth of'50 mm andan end angle of 60C and an attempt to convert said polycrystal into single crystal was made by the Bridgman's method at a cooling rate of 2C/hr. The crystal in the crucible was not converted into single crystal, but into some crystal masses sticking to platinum-wall.

c. In order to grow a thin crystal plate of MOG, a platinum vessel of 10 mm in width, 15 cm in length and 5 cm in depth was filled with polycrystalline powders of MOG. Then, the polycrystalline powders' were molten and thereafter cooled at various temperature gradients to obtain no single crystal. Especially, the whole surface of crystal did not become horizontal plane and meniscus due to surface tension was apt to increase.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing single crystal, which is not effected by the defect that a melt of MOG will wet a platinum vessel and has a high sticking force to the surface of platinum when it solidifies.

Another object of the present invention is to provide a method for optionally producing single crystal plate of MOG having a desired specific crystal.

That is, the gist of the present invention resides in that while keeping the whole melt of MOG at molten state the, temperature of only the surface layer is rapidly reduced to lower than the melting point to grow single crystal plate in the surface layer. After single crystal layer is formed in the surface of the melt by said method, temperature of the whole melt is lowered to solidify it and it is cooled to room temperature and thereafter said single crystal plate is peeled or single crystal seed of MOG or a platinum rod is introduced into the melt to allow crystal plate to grow and then the crystal plate may be drawn up from the melt.

The method of the present invention does not comprise cutting a crystal plate of a desired orientation (especially 001 direction), but comprises previously and directly producing a crystal plate of a desired orientation and is conspicuously excellent as compared with the conventional method with reference to time and mass production.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a sectional view of a heating furnace which explains essential parts of the heating furnace used for heating gadolinium molybdate in the present invention.

FIG. 2 is a graph which shows the light transmission characteristic of single crystal plate of MOG at various wave lengths.

FIG. 3a shows one embodiment of the method for producing single crystal plate of MOG of the present invention.

FIG. 3b shows a sectional view of a heating furnace which is used for producing single crystal plate of MOG of the present invention.

. FIG: 4 is a characteristic-curve which shows correlation between depth. of MOG melt and success rate of .single crystal plate obtained.

' FIG. 5 is a'characteristic curve which'shows correlaplate obtained.

' FIG. 6 is a characteristic diagram which shows reduction. rate of furnace temperature in production of single -crystal plate of MOG of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention will be explained by way of Examples.

EXAMPLE I 120 Grams of sintered MOG was placed in a platinum vessel having a diameter of 50 mm and a depth of 30 mm and was heated in a muffle furnace 1 as shown in FIG. I. Said muffle furnace was provided with carbon siliside heater 2 which was a main heater and subheater 3 of a platinum heater which corrects temperature distribution. These two heaters can independently adjust the temperature and the main heater can effect program control. Temperature of the furnace was raised to l200C at a rate of about 300C/hr until it reached l000C and at 100C/hr until it reached l200C. The furnace was kept at l200C for about one hour. Thereafter, while keeping such temperature distribution as the temperature of the bottom of the vessel in FIG. 1 was higher than that of the upper part by about 10C, the temperature of the whole of the vessel was reduced at 100C/min until llC, at 50C/hr from ll00C to l000C, at 200C/hr from l000C to 250C, at 50C/hr from 250C to 100C and by leaving it for cooling until room temperature. As the result, a solid of MOG in a thickness of about 10 mm was formed in a platinum vessel and it was acknowledged that the surface of the solid was single crystal. Removal of the platinum vessel showed that the solid comprised the upper part of thin single crystal layer and the lower part of polycrystalline layer. From the single crystal layer, a single crystal plate having a side of 5 mm-l0 mm and a thickness of 200-500 u. could be peeled off. It was found that this single crystal plate was 00l plate.

According to observation with a microscope, the surface of the single crystal plate had a part containing circular convexes of -50 1.1. in thickness beside smooth part.

A smooth crystal plate could be produced by polishing both surfaces of the crystal plate with No. 2000 alumina powders. The light transmission characteristic of thus obtained crystal plate is as shown in FIG. 2.

EXAMPLE 2 100 Grams of MOG was introduced into a platinum vessel which had a bottom of 3 cm X 5 cm and a depth of 1 cm and upwardly tapered by V4 and was heated in the same furnace as used in Example 1 except that the subheater was reconstructed in such a manner that the platinum heater was wound in closely two steps, whereby a temperature gradient along the long side of the vessel of 2-l0C/cm at l200C was obtained. Furthermore, a temperature gradient of about C between the bottom (the high temperature part) of the vessel and the upper part (the lower temperature part) was imparted. The temperature of the furnace was elevated to l200C at about 300C/hr until l000C and at 100C/hr from l000C to l200C and kept at 1200C for about one hour. The temperature of said melt was lowered to 1 100C at l00-500C/35 min while keeping said temperature gradient and furthermore lowered to l000C at 50C/hr, to 250C at 200C/hr and to lower than 250C by leaving it as it was. As the result, a solid layer of MOG was formed in the platinum vessel and the surface of the melt was grown to single crystal except for the part contacting with the edge of the vessel. Removal of the platinum vessel showed that the solid layer comprised the upper thin single crystal layer and the lower polycrystalline layer and a single crystal plate having a side of 5 mm and a thickness of 200-500 p. was obtained from said single crystal layer. This single crystal plate had a surface having less circular convexes than the single crystal plate obtained in Example 1. The light transmission characteristic curve thereof substantially corresponds to curve 4 in FIG. 2.

EXAMPLE 3 220 Grams of sintered MOG was introduced into a platinum vessel of 50 mm in diameter and 40 mm in depth. This vessel was placed in a work coil of mm in diameter and of nine turns which was connected with a high frequency oscillator of 400 KC and 8 KW, in such a manner that the bottom and side face of said vessel were kept warm with a heat insulating material. The vessel was heated by high frequency induction heating to melt MOG in the vessel at l200C to obtain a melt. A piece of single crystal of MOG as seed crystal was contacted with the surface of the melt using a crystal drawing up axis of a single crystal drawing up device for about 10 minutes and then high frequency current was abruptly decreased to cause immediate growth of crystal plate from the point contacting with the seed crystal. Before the growing crystal plate contacts with and sticks to the wall of the vessel, high frequency current was increased to raise the temperature and simultaneously the seed crystal and the crystal plate grown therefrom were slightly drawn up from the surface of the melt to cause no contact with the surface of the melt. At this state, they were cooled to about l000C at 100C/hr, to 250C at 500C/hr and finally to room temperature by leaving it as it was. As the result, the crystal plate grown from the seed crystal contained several single crystals and a single crystal plate having a side of 8-5 mm and a thickness of 200-300 s was taken out. According to observation with a microscope, the crystal included some precipitation and dendritic growth and was inferior to that obtained in Examples l and 2 in its homogeneity.

In place of said seed single crystal, a platinum rod may be used.

EXAMPLE 4 l2 Grams of sntered GMO was introduced into a platinum boat having a circular bottom and a length of 4 cm, a width of 1 cm and a depth of 1 cm and said boat 30 was covered with a heat insulating material 30' as shown in FIG. 3a, and fixed in a quartz tube 32 of 4 cm in diameter in tubular furnace as shown in FIG. 3b and then said boat was heated in the said tubular furnace 31. The temperature was elevated to l200C to melt MOG in the boat and then a dehydration-dried air was passed through said quartz tube in such a manner that amount of the air was small at first and gradually increased until 3 I/min. The temperature around the MOG was also raised to 1250C, which was kept by controlling the furnace. When the flow amount of air reached 3 l/min, this state was kept for one hour. While keeping air flow at said state, the temperature was lowered to l000C at 50C/hr and then to 500C at 200C/hr. When the temperature reached 500C, air flow was discontinued and the temperature was further lowered to 250C at 200C/hr, to 100C at 50C/hr and finally to room temperature by leaving it as it was.

The surface of MOG solidified in the platinum boat was partially single crystallized and said partial single crystallized plate was removed from the platinum boat.

The said removed single crystal plate was 4 mm in length, l-2 mm in width and 200-400 p. in thickness.

As shown in the above Examples, according to the present invention, normal temperature gradient (i.e., high temperature at inner part of melt and lower temperature at surface layer) is given to the surface of the melt of MOG to produce a single crystal layer of OOl direction in the surface layer of the melt. in order to especially effectively carry out, the following procedures are preferable.

1. The thickness of the MOG melt is required to be more than 5 mm, actually more than mm. The relation between the thickness of the melt (depth in mm) and success rate of single crystal plate obtained (expressed by percent) is shown by the characteristic curve 5 in HQ. 4. As is clear from the curve, success rate ofsingle crystal obtained when depth of the melt is more than about 30 mm is high.

2. Furthermore, relation between the size of crucible containing the melt and success rate of single crystal plate obtained is shown by the characteristic curve 6 in FIG. 5. According to this curve, the shape of the vessel may be circular or square one and diameter in case of circular vessel and side in case of square vessel (i.e. cross section) should be more than mm.

3. As the method for growing crystal plate only in the surfacelayer, there are the following methods.

a. The method according to which the temperature is abruptly lowered at about melting point.

b. The method according to which a gas whose temperature is lower than ambient temperature is passed on the surface of the melt.

In case of '(a), it is necessary to lower the temperature of around the surface by about more than 100C at a stroke. When the reduction of the temperature is small, a complicated temperature gradient is resulted inwardly to the melt, whereby a thick growth of crystal layer is caused and eventually growth as single crystal is discontinued. Actually effective range is l00-500C. When the reduction of the temperature is great, formation of dendrite is observed in crystal plate and crystallinity becomes poor. Effective temperature reduction to about l200-1 100C is within the range as shown by oblique lines in FIG. 6.

4. After growth of crystal plate, the temperature of the whole melt is lowered to solidify remaining melt below the crystal plate and the melt is slowly cooled.

5. In order to maintain quality of the crystal plate, at-

tention should be given to cooling rate when the crystal plate is at the transition point of 160C. When the cooling rate is high at the transition point, a number of cleavages in the direction of l 10 are produced in crystal plate face and no large crystal plate can be obtained. Therefore, the crystal plate should be cooled at a cooling rate of less than 50C/hr during the period of from 300C to The term success rate used herein means yield of single crystal plate.

The term No. 2000 alumina powders used for polishing crystal plate in Example 1 means alumina powders having a diameter of 3 M.

What is claimed is:

1. A method for producing a single crystal plate of gadolinium molybdate which comprises heating gadolinium molybdate in a vessel to 1,200C thereby forming a melt, providing a temperature gradient between the upper and lower portions of the melt contained in the vessel, the upper portion being at a lower temperature than the lower portion, cooling the gadolinium molybdate melt at a cooling rate of 30C/min. to 170C/min. to a temperature of l,l00C while maintaining the temperature gradient to obtain a single crystal plate in the surface, further cooling the gadolinium molybdate melt at a cooling rate slower than the cooling rate between 1,200C and l,l00C to room temperature thereby obtaining solid gadolinium molybdate wherein the surface thereof is a single crystal plate and the remaining portion is polycrystalline gadolinium molybdate, and removing the single crystal plate from the polycrystal.

2. The method for producing a single crystal plate of gadolinium molybdate according to claim 1, wherein the thickness of the melt formed is more than 10mm.

3. The method for producing a single crystal plate of gadolinium molybdate according to claim 2, wherein the vessel used has a cross section which is more than 20mm.

4. The method for producing a single crystal plate of gadolinium molybdate according to claim 1, wherein the single crystal plate of gadolinium crystal is produced while passing a gas having a temperature lower than the environmental temperature of the melt to the surface of the melt.

5. The method for producing a single crystal plate of gadolinium molybdate according to claim 2, wherein the single crystal of gadolinium crystal plate is produced while passing a gas having a temperature lower than the environmental temperature of the melt to the surface of the melt.

6. The-method for producing a single crystal plate of gadolinium molybdate according to claim 3, wherein the single crystal plate is produced while passing a gas having a temperature lower than the environmental temperature of the melt to the surface of the melt.

7. The method for producing a single crystal plate of gadolinium molybdate according to claim 1, wherein the temperature gradient is 10C.

8. The method for producing a single crystal plate of gadolinium molybdate according to claim 1, wherein the temperature gradient is 20C.

9. The method for producing a single crystal plate of gadolinium molybdate according to claim 1, wherein a 001 crystal plate is formed.

10. The method for producing a single crystal plate of gadolinium molybdate according to claim 1, wherein the amount of gadolinium molybdate present is sufficient to produce a melt having a thickness of more than 30 mm.

11. The method for producing a single crystal plate of gadolinium molybdate according to claim 1, wherein the rate of cooling between 1,200C and l,l00C is C/min.

12. The method for producing a single crystal plate of gadolinium molybdate according to claim 1, wherein the gadolinium molybdate melt is cooled at a rate of 50C per hr. between 300C and 100C. 

2. The method for producing a single crystal plate of gadolinium molybdate according to claim 1, wherein the thickness of the melt formed is more than 10mm.
 3. The method for producing a single crystal plate of gadolinium molybdate according to claim 2, wherein the vessel used has a cross section which is more than 20mm.
 4. The method for producing a single crystal plate of gadolinium molybdate according to claim 1, wherein the single crystal pLate of gadolinium crystal is produced while passing a gas having a temperature lower than the environmental temperature of the melt to the surface of the melt.
 5. The method for producing a single crystal plate of gadolinium molybdate according to claim 2, wherein the single crystal of gadolinium crystal plate is produced while passing a gas having a temperature lower than the environmental temperature of the melt to the surface of the melt.
 6. The method for producing a single crystal plate of gadolinium molybdate according to claim 3, wherein the single crystal plate is produced while passing a gas having a temperature lower than the environmental temperature of the melt to the surface of the melt.
 7. The method for producing a single crystal plate of gadolinium molybdate according to claim 1, wherein the temperature gradient is 10*C.
 8. The method for producing a single crystal plate of gadolinium molybdate according to claim 1, wherein the temperature gradient is 20*C.
 9. The method for producing a single crystal plate of gadolinium molybdate according to claim 1, wherein a 001 crystal plate is formed.
 10. The method for producing a single crystal plate of gadolinium molybdate according to claim 1, wherein the amount of gadolinium molybdate present is sufficient to produce a melt having a thickness of more than 30 mm.
 11. The method for producing a single crystal plate of gadolinium molybdate according to claim 1, wherein the rate of cooling between 1,200*C and 1,100*C is 100*C/min.
 12. The method for producing a single crystal plate of gadolinium molybdate according to claim 1, wherein the gadolinium molybdate melt is cooled at a rate of 50*C per hr. between 300*C and 100*C. 